1. Field of the Invention
The present invention relates to a photoelectric conversion element, an image reading device, an image forming apparatus, and a signal control method.
2. Description of the Related Art
An image reading device, such as a scanner, performs photoelectric conversion on light reflected by a document, and performs processes, such as amplification or an analog-to-digital (A/D) conversion, on an analog image signal obtained through the photoelectric conversion. Conventionally, a charge coupled device (CCD) has been mainly used as a photoelectric conversion element. However, with a recent demand for lower electric power, a complicated metal-oxide semiconductor (CMOS) linear sensor has attracted attention. The CMOS linear sensor is the same as the CCD in that it uses a photodiode (PD) to perform photoelectric conversion on incident light, but differs in that it performs a charge-voltage conversion in the vicinity of a pixel and outputs a voltage to a subsequent stage whereas the CCD first transfers electric charges by using a shift register and thereafter performs a charge-voltage conversion by an electric-charge detecting unit. Further, the CMOS linear sensor can be manufactured by a CMOS process, so that the CMOS linear sensor may include a built-in logic circuit, a built-in programmable gain amplifier (PGA), and a built-in A/D converter (ADC).
Furthermore, in recent years, a column system for increasing a driving speed has been known, in which a plurality of pixels (for example, three pixels of red, green, and blue) are treated as a single pixel group (column), analog processing circuits, such as a PGA and an ADC, are provided for each column to enable parallel processing, and each of the analog processing circuits sequentially performs analog processing on analog image signals read from the plurality of the pixels in the column (for example, in order of red, green, and blue).
Moreover, Japanese Laid-open Patent Publication No. 2011-029793 discloses a solid state imaging device, which includes a line (column) of unit pixels and includes a noise correction circuit that corrects, for each row, a detected pixel signal by using a noise component.
However, in the conventional CMOS linear sensor of the column system, the operating state immediately before processing varies between the first pixel (for example, a red pixel) that is first subjected to analog signal processing in a column and other subsequent pixels (for example, green and blue pixels), so that a characteristic difference occurs between the pixels (between colors) resulting in deterioration of image quality (coloring or false color).
Further, power, a ground voltage GND, and a reference voltage are supplied to the PGA and the ADC, and load fluctuation or a switching noise occurs in the PGA and the ADC when operation is performed. In this case, the operating state immediately before processing varies between the first pixel and the other subsequent pixels, so that the load fluctuation or the switching noise varies, resulting in a gain error between the pixels (between the colors) in the PGA and a conversion error between the pixels (between the colors) in the ADC. If there is linearity in the characteristic difference, it may be relatively easy to correct an image signal at a subsequent stage and a problem may not occur. However, it is difficult to correct complex characteristics, such as a characteristic difference that varies depending on pixel signal levels or the columns, at a subsequent stage. Therefore, the image quality may be deteriorated (the characteristic is not uniform between the colors and coloring or false color occurs), or a chip size or cost may increase by providing a dedicated correction circuit.
In view of the above circumstances, there is a need to provide a photoelectric conversion element, an image reading device, an image forming apparatus, and a signal control method capable of easily preventing deterioration in the image quality due to an analog processing unit that sequentially processes, for each of pixels, analog signals output from a pixel group including the pixels, without providing a dedicated correction circuit or the like.